In recent years, there has been increasing insistance on the part of federal and local governments for municipalities and industries to pretreat their used water and remove all pollutant and contaminants before recycling the water to the hydrogeological environment. As more used water-treatment plants come into existence and as their efficiency for removing dilatorious matter improves, a secondary problem of major proportions develops, which relates to the ultimate disposal of the solid material precipitated from the used water-treatment plant.
The solid materials collected from such treatment facilities have potential as a valuable recyclable natural resource; however, its collected form is not marketable. Until the turn of the mid century, natural organic materials (now called waste products) accounted for a substantial portion of the fertilizer nitrogen supply, while today, such materials supply less than 1% of the total market. The reason is that modern-day fertilizers are high in fertilizer nitrogen, granular and are odor-free, while natural organic materials are often low in nitrogen, are dusty and often given to odor problems.
Solid material collected from waste water-treatment plants is normally referred to as sewage sludge, and in its final form, is a filter cake containing between about 12 to 50% solids, with the balance water.
In a standard waste water treatment, sewage influent enters a primary sedimentation or precipitation tank where the bulk of the solids is collected and transferred to a digestor for action upon by an anaerobic bacteria for a period of from one to three months, depending on the environmental conditions set up in the digestor. A by-product of the digestor is methane gas which can be recovered and may be used as a source of energy.
The resulting digested sludge (containing about 92 to 95% water) is normally sent to a dewatering process where the moisture is reduced to between about 70 and 50%. In some cases, biological sludges will contain as much as 88% moisture and only 12% solids after dewatering. Various methods have been used for this dewatering process, such as: centrifuging, vacuum filter wheels and sand filter beds. All these methods have certain advantages which may designate their use in a particular area. However, they all have one common problem: a wet filter-cake material which has no utilitarian value in its present physical form, and an infectious disease problem.
In order to enhance and to fortify the fertilizer value of fertilizer sludge and various other carriers, urea and formaldehyde have been incorporated therein by various techniques and under different conditions (see, for example, U.S. Pat. Nos. 3,073,693; 3,226,318; 3,231,363; and 3,655,395). However, these processes do not relate to techniques for directly converting sludge filter cake into a sterilized, granular, high-nitrogen, dust-free product suitable for use as a fertilizer or an animal feed supplement product.